FIG. 1 shows a cross section of a typical prior art high voltage Silicon On Insulator (SOI) device. The device 100 in FIG. 1 includes the typical components and regions thereof, such as the source 101, drain 103 and gate region 102. The buried oxide layer 104 forms a junction with the SOI layer 105, both disposed above the substrate layer 106. Such devices are, in many applications, utilized in a bias condition wherein the source 101, substrate 106, and gate 102 are maintained at ground potential, while the drain region 103 is maintained at or near 200 volts. Such bias conditions are typical in a variety of switching applications relating to medical and communications technology, as well as other technological fields.
Such a device can easily support the 200 volts applied to the drain in an off state with a drift length of approximately 12 micrometers.
Although the configuration of FIG. 1 works well when the device is biased as described, various applications require a different type of biasing. More specifically, in certain switching operations it is necessary to apply a relatively large positive voltage to the drain, and a relatively large negative voltage to the source and gate, while maintaining the substrate voltage at approximately ground. In such configurations, it is typical to bias the source and gate at approximately negative 100 volts, the drain at approximately positive 100 volts, and the substrate layer at approximately 0 volts. This bias condition is referred to as source below substrate potential. Such a condition is employed in a wide variety of circuitry with telephony, medical, and other applications. In such a condition, the substrate is maintained at a voltage between that of the source and drain, rather than the substrate being maintained at a voltage substantially the same as that of the source, as was described with respect to the prior art.
FIG. 3 shows the simulation of the device of FIG. 1 biased in a source below substrate bias condition. Although an attempt is made to bias the source at negative 100 volts and the drain at plus 100 volts, the device can only maintain a maximum difference of approximately 65 volts between the source and drain. The problem is that the breakdown voltage of the device falls from over two hundred volts when it is biased as described with respect to the prior art, to approximately 65 volts when the biasing condition is changed to source below substrate. Accordingly, there exists a need in the art for a thin layer SOI switching device that can operate in the source below bias condition without suffering breakdown at a relatively low voltage.